Because of their light weight, atmospheric corrosion resistance and high strength-to-weight ratio properties, aluminum alloys are popular materials of construction and many different alloy compositions are well known in the art. A system of four digit numerical designations has been established to identify these aluminum alloys. The first digit signifies the primary alloying elements, while the other digits signify a particular grade or product form.
A popular class of alloys in this classification system is the 3XXX series, of which the 3003 and 3010 grades are representative. These alloys contain nominal amounts of manganese and magnesium, and are popular due to their relatively low cost, their ability to be easily cast or worked, and their mechanical properties (i.e., tensile and yield strengths), which are satisfactory for certain applications. In many situations, however, the 3XXX series does not provide sufficient corrosion resistance, particularly against solutions that cause pitting.
Pitting or pitting corrosion is the localized attack of a metal surface which is confined to a small area and which takes the form of cavities. The depth of these cavities can range from a few microns on the surface to throughout the entire thickness of the metal. Pitting is a particularly troublesome type of corrosion because, although most of the metal is not attacked, the deeper pits seriously weaken the metal and often cause premature failure of the part. While pitting corrosion is detrimental to any metal or finished part, it is a much greater concern when the metal has been fabricated or processed into thin shapes or gauges.
Pitting corrosion resistance can be improved by resorting to a higher alloy content composition, but in addition to increased cost, these higher alloys are more difficult to cast or fabricate into shapes.
When utilizing aluminum alloys in the form of thin shapes or small parts, there are many applications where increased mechanical properties would be beneficial or necessary. This can also be resolved by the substitution of a higher alloy composition, but, again, higher costs and fabrication difficulties will be encountered.
The present invention overcomes the deficiencies of the 3XXX series while avoiding the disadvantages of the higher alloy alternatives. Through a unique combination of small amounts of alloying elements, the compositions claimed in this invention provide substantially improved pitting corrosion resistance compared to the prior art.
An additional advantage of the aluminum alloy compositions of the present invention is better mechanical properties compared to the 3XXX series while retaining similar casting and working abilities. The aluminum alloys of the present invention can be readily fabricated by casting and either hot or cold rolling to thin gauges. They can also be easily formed into shapes by drawing, stamping, or extruding.
Due to their tolerance for certain levels of impurity or tramp elements, the cost of manufacture of these compositions is relatively low and compares favorably to the cost of the 3XXX series alloys.